Exactly 100 years ago, Secretary of State Philander Knox certified that the 16th Amendment - granting Congress the explicit authority to levy a tax on income "from whatever source derived" - had, in fact, won approval from 36 states (the magic number in those days).

Ratification, however, did not produce an income tax, at least not right away. In fact, it took Congress almost eight months to exercise its new-found authority. The first income tax of the 20th century made its way into law as part of the Underwood-Simmons Tariff Act, passed on October 4, 1913.

Just to make matters more confusing for those who love a good tax party, Congress made the 1913 law retroactive, applying it to income earned after March 1, 1913.

So depending on how you do your counting, the income tax won't turn 100 for another 8 days. Or maybe another 220. You make the call.

While we're at it, there are a three other things to know - or not know - about the centennial of the 16th Amendment amendment:

1.The amendment probably wasn't necessary.

In 1894, Congress had passed a modest income tax, and almost immediately, the Supreme Court had struck it down in the decision for Pollock v. Farmers' Loan & Trust Company.
Fifteen years later, income tax supporters introduced the the 16th Amendment to fix that problem. But plenty of people thought an amendment was unnecessary. The Court (and the country) had changed a lot since Pollock, and many observers thought the justices would reverse themselves if given another bite at the apple. After all, observed Sen. Joseph Bailey.at the time, "An overwhelming majority of the best legal opinion in this Republic believes that Pollock was erroneous."

Speaking of tax protesters, they've developed a body of pseudo-serious scholarship arguing that the 16th amendment was never actually ratified. (Erroneusly assuming that non-ratification would make the current income tax unconstitutional; see above). But the protester arguments have been repeatedly and firmly dismissed by the courts. In particular, the courts to have rejected arguments that some states supposedly passing the amendment had in fact done no such thing. And they have also dismissed the notion that minor changes in the wording of the amendment, as considered by various state legislatures, made ratification impossible.

Knox, incidentally, considered these discrepancies when deciding to certify ratification, and he went ahead anyway. Making that sort of judgment call is pretty much the purpose of certification in the first place.

3. The Secretary of State doesn't certify amendments any more.

In 1951, that responsibility was transferred to the General Services Administration and in 1984, to the newly independent National Archives (part of the GSA until that point).